1. Field of the Invention
The present invention relates generally to a weft sensor for a loom and more specifically to a weft sensor having a plurality of weft-detection means for reliably detecting the presence or absence of a weft inserted into a warp shed of a loom.
2. Description of the Prior Art
The background of the present invention will be explained with respect to its application to an optical weft sensor for an air-jet loom, by way of example.
As is well known, an optical weft sensor is provided for an air-jet loom in order to optically detect that a weft thread is securely inserted into air-guide plates having an air-guide opening and a weft-removing slot respectively and arranged in the direction of weft insertion. The optical weft sensor comprises a light-emitting section and a light-receiving section for detecting the presence or absence of weft in dependence upon the change in magnitude of the received light, which is caused when a weft is removed through the weft-removing slots of the air guide plates and across an optical axis formed between the light-emitting section and the light-receiving section during the beat-up stage. Further, the optical weft sensor is usually attached to one air-guide plate fixed at the one end of the reed holder disposed at a position remote from a fluid nozzle and the loom is stopped when the weft sensor detects the absence of a weft thread.
In the prior-art optical weft sensor, however, there exist some problems or shortcomings as follows:
Since the diameter of a weft is small and additionally fluff occasionally attaches to the weft-removing slot of the air-guide plate, the detected signal voltage level indicative of the presence of a weft is relatively low and noise signal level due to the presence of fluff is relatively high.
Additionally, since a pair of light-emitting and light-receiving sections (referred to as optical weft-detection means) are fixed to the air guide plate and therefore the optical weft-detection means is moved to and fro all the time during the beat-up motion, the lead connected between the optical weft-detection means and the detection circuit is alway moved to and fro and therefore the lead tends to be frayed (some parts of a plurality of copper wires are broken off). If the lead is completely broken off, since the detection circuit can stop the loom, no serious problem arises; however, in case the lead is frayed, that is, in case the lead is imperfectly connected, the lead will generate noise signals which make even more difficult the accurate detection of the presence of a weft thread.
Therefore, in the case where only a single optical weft-detection means having a pair of light-emitting section and light-receiving section is provided for detecting the presence or absence of a weft inserted into the openings of the air-guide plates, it is very difficult to adjust the sensor sensitivity at an acceptable moderate level. To explain in more detail, if the sensor sensitivity is too high, when fluff attaches to the weft-detection means or the sensor lead is imperfectly connected, the sensor will detect this state as the presence of a weft and keep the loom operating continuously, in spite of the fact that a weft is not inserted, thus causing the deterioration of quality of woven cloth (referred to as erroneous weft-presence detection due to high sensitivity). In contrast with this, if the sensor sensitivity is too low, whenever a slender weft thread is inserted, the sensor will detect this state as the absence of a weft and stop the loom unnecessarily, in spite of the fact that a weft is inserted, thus causing the lowering of loom duty cycle (ratio of time in use to total time) (referred to as erroneous weft-absence detection due to low sensitivity).
In the case where a single light-emitting section is provided on one side of the weft inserting channel at the opposite end of the weft inserting nozzle and a plurality of light-receiving sections are provided on the other side thereof, being connected to an OR gate, for detecting the presence or absence of a weft, since the loom is stopped only when all the light-receiving sections detect the absence of the weft, if fluff attaches to the light-receiving section, the sensor will detect this state as presence of a weft and keeps the loom operating, thus causing the deterioration of woven cloth.
The arrangement of the latter prior-art optical weft sensor for a loom will be described in more detail hereinafter with reference to the attached drawing under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.